Generally, a multilayer structure obtained by laminating layers of saponified ethylene-vinyl ester-based copolymer resin (EVOH resin) is used for giving gas barrier properties to films or containers made of thermoplastic resin such as polyester or polyolefin.
Adhesiveness of EVOH resin to thermoplastic resin such as polyester or polyolefin is low. Accordingly, in manufacturing a multilayer structure containing thermoplastic resin layer and EVOH resin layer, normally, an adhesive resin layer (normally using acid anhydride modified resin) is interposed between these layers.
However, in a certain molding method, it may be difficult to interpose an adhesive resin layer between an EVOH resin layer and a thermoplastic resin layer. As for a coinjection molding machine, a molding machine using two types of materials is mainly used. Accordingly, for instance, in the case where PET bottles having a 2-type 3-layer structure such as PET/EVOH/PET with gas barrier properties are manufactured by coinjection molding, it is difficult to interpose, between the PET layer and the EVOH layer, an adhesive resin layer made of other resin material, in view of the performance of the molding machine. Thus, there is a demand for EVOH resin capable of providing a multilayer structure having excellent delamination resistance, without interposing an adhesive resin layer.
Improved adhesiveness to thermoplastic resin has been found in an EVOH resin composition containing specific metal in EVOH resin. Use of the EVOH resin composition enables to manufacture a multilayer structure containing layers of thermoplastic resin such as polyester, without using an adhesive resin layer. Thus, research and development have progressed for manufacturing such an EVOH resin composition.
For instance, JP 2005-82226A (patent document 1) proposes a container manufactured by subjecting a resin composition containing a salt, an oxide, or a hydroxide of at least one kind of metal selected from the group consisting of zinc, nickel, iron, chrome, vanadium, calcium and magnesium, in a concentration of 0.001 to 1 wt % (10 to 10,000 ppm) relative to EVOH resin, and thermoplastic polyester to coinjection stretch blow molding. Specifically, a container manufactured by subjecting an EVOH resin composition added with 0.088 wt % magnesium acetate tetrahydrate or 0.022 wt % zinc acetate dehydrate, and polyester to coinjection stretch blow molding has a low delamination resistance against impact, as compared with a container manufactured by coinjection stretch blow molding with use of EVOH resin without adding these acetate metal salts (see Table 1).
Further, JP 2005-89482A (patent document 2) discloses a blow-molded container composed of, as an EVOH resin composition for use in a 2-type 3-layer multilayer structure of PET/EVOH/PET by coinjection molding, an EVOH resin composition containing 0.1 to 20 μmol/g alkali metal salt in terms of alkali metal; 0 to 2 μmol/g carboxylate radical (C1) extracted by 10-hour immersion in water at 95° C.; and 0 to 40 μmol/g carboxylate radical (C2) extracted by 10-hour immersion in an aqueous solution containing 0.05 mol sodium hydroxide at 95° C. Specifically, patent document 2 describes that with use of EVOH resin (example 1) containing, in EVOH resin composition pellets, 3.40 μmol/g potassium salt in terms of potassium, 1.2 μmol/g phosphate compound in terms of phosphate radical, and 0 ppm carboxylate radical (C1) obtained by isolation/extraction by 10-hour immersion in hot water at 95° C., no hard spots and no coloration were observed in the containers (bottles), even after the 2-type 3-layer multilayer structure was continuously subjected to coinjection blow molding for 72 hours (see paragraphs 0119, 0120 and Table 2).    [Patent document 1] JP2005-82226A    [Patent document 2] JP2005-89482A (counterpart US application publication No. 2005038163)